Low temperature photoluminescence from disordered granular ZnO

In an effort to shed light on the widely discussed issue of defects and photoluminescence (PL) from ZnO material, we have undertaken this in-depth study using low temperature PL and room temperature Raman spectroscopy. Initial defective states of polycrystalline ZnO have been varied by annealing at 500 and 800 °C. Further, point defects have been incorporated in the systems by 700 keV oxygen ions. 10 K PL spectra of all the unirradiated and irradiated samples show dominant donor bound exciton at 3.365 eV. Based on the earlier reports, this shallow donor has been assigned as interstitial zinc (IZn) in ZnO. It is found that the PL intensity variation of 3.365 eV PL with irradiation is different for 500 and 800 °C annealed samples. This donor has been found to be responsible for the red PL (~1.9 eV) at cryogenic temperatures (only for the sample irradiated at moderate fluence, 3×1014 ions/cm2). Specific defect configuration involving IZn and a zinc vacancy, both stabilized by oxygen vacancies, have been proposed as the origin of ~1.9 eV red PL. 500 °C annealed sample becomes brown-black after irradiation at highest fluence (3×1016 ions/cm2) showing lower resistance compared to the pristine one. The Raman spectrum of this particular sample shows loss of long range crystalline order although IZn related PL emission (at 10 K) is highest here. In the light of combined experimental outcome and understanding, an illustration on ion beam irradiation effects in granular ZnO has been presented at the end.